Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

72 S. K. Singhfrom Uppsala. We also used GSttingen seismograms/bulletins to estimate magnitudes.Recall that Ms computed from (1) is 0.18 unit less that the Ms reportedin Earthquake Data Report (EDR) of U.S. Geological Survey (Abe, 1981). Ourcatalog of shallow Mexican earthquakes (Singh et al., 1984a) lists Ms so that itconforms to EDR. Tests show that it is complete for MS 2 6.5 from July 1906 toDecember 1981 (Singh et al., 1984a).Abe and Noguchi (1983) have further revised Ms values of large earthquakes inthe period 1897-1912. This revision does not change equation (l), hence our catalog,significantly since only 3 events shown in Figure 1 have been assigned different Ms(on an average lower by 0.2 unit).3. Statistics of Small Earthquakes and Frequency of Occurrence of LargeEarthquakes: Evidence of Characteristic Earthquakes in Mexico.Singh et al. (1981) found that Gutenberg-Richter (GR) relation, log N = a-bMs,was not valid for Mexico for Ms 2 7.0; there was a deficiency of events for 7.0 5Ms 5 7.4 with respect to what one would expect from the GR relation. This resulthad great implications for seismic risk studies. It also implied that areas rupturedpreferentially with certain maximum magnitude. With the completion of the newcatalog the earthquake statistics were reexamined. Figure 2 shows 4 regions inwhich the subduction zone of Mexico was divided (Singh et al., 1983). The plot oflog N - Ms is shown in Figure 3 for the combined regions of Oaxaca, Guerrero,and Michoacdn (Jalisco region was excluded because the seismicity in this regionis related to the subduction of the Rivera plate and is expected to differ from theother 3 regions). In Figure 3, 75.5 yr data, taken from the catalog, as well as 18.5 yrdata (since the installation of WWSSN) normalized to 75.5 yr, are plotted. For thisFigure 2. Regions along the Mexican subduction zone studied for the statistics ofearthquakes. Seismicity of Oaxaca, Guerrero, and Michoacin regions is related to thesubduction of the Cocos plate whereas the seismicity of Jalisco is mostly related to thesubduction of the Rivera plate below Mexico. (From Singh et al., 1983).